This invention is in the field of electrophotography and relates to a method for electrostatic recording on a cylindrical photoreceptor with dielectric coating and an electrophotographic apparatus for multiple-copy production utilizing this method. The invention is particularly useful in the creation of large-format copying equipment, as well as highly productive equipment for the manufacture of printed circuit boards (PCBs) and polygraphic matrices.
The electrophotographic process is predominantly used for easy and quick image reproduction or copying. The ability of technology to reproduce/copy an image onto different materials serving as the final information carrier enables the use of electrophotographic methods of copying in different technical fields. One of these fields is the field of printed circuit boards (PCBs) production.
The PCB production process is the process of creating a pattern of conductors on a carrying dielectric basis. The traditional process of PCB production typically consists of the following main stages:
(1) depositing a special photoresist material onto the copper-foiled surface of a plate;
(2) applying optical means to form a required conductive pattern which is created due to the polymerization of the photoresist material within the image regions;
(3) removing the photoresist material from the non-exposed regions (i.e., spaces between the image regions);
(4) applying chemical etching to remove the copper foil from the non-exposed regions;
(5) removing the photoresist material from the image regions (stripping the current conductors).
There exists a number of technical solutions for pattern creation on a foil-coated plate by means of electrophotography. These solutions utilize either direct or indirect processes of electrophotographic copying. For example, U.S. Pat. Nos. 4,327,167 and 5,494,764 disclose a technique of PCBs production by means of a direct method of electrophotographic copying. For this purpose, a photo-semiconductive film is preliminary deposited on the foiled surface of a plate, which thereby is transformed into a photoreceptor. The printing process comprises the stages of electrostatic charging, exposure, development of a latent electrostatic image by a toner, and the toner fixation on the surface of the photo-semiconductive layer. Thereafter, the photo-semiconductive material is removed from the blanked regions, and selective chemical etching and stripping of the current conductors are carried out. It is evident that such cumbersome and relatively expensive technology has no essential advantages over the conventional one.
The application of the indirect process of electrophotographic copying using a photoreceptor as an intermediate image carrier is more effective. However, in this case, other problems arise associated mostly with the ability for high-quality reproduction of large-format images and the realization of a process of toner-image transfer from the photoreceptor surface onto a conductive information carrier. It is evident that a method of electrostatic transfer in the corona-discharge field which is widely used in the electrophotographic technique, as well as other known methods of the toner-image transfer onto a dielectric information carrier (paper, film, etc.) are not applicable for the manufacture of PCBs. For this reason, the known techniques of the kind specified utilize the adhesion-based transfer, wherein the forces of toner adhesion to the surface of a receiving material should exceed the forces of electrostatic attraction that hold the toner particles on the surface of the photoreceptor. These techniques are disclosed for example in U.S. Pat. Nos. 4,371,599; 5,470,644 and 5,576,135.
According to the technique disclosed in U.S. Pat. No. 5,470,644, the direct, adhesive toner transfer onto the PCB surface is used, wherein the PCB surface is coated with ink aimed at improving its adhesion properties. However, this technique suffers from the fact that the processes of preliminary PCB coating and subsequent ink removal before etching still remain. Additionally, this technique practically does not provide the full transfer of a toner-image, which may lead to intolerable defects in the image causing discontinuities in the conductive elements of the PCB.
The above drawbacks are eliminated to a considerable degree, when using a thermal method of the adhesive transfer of a thermoplastic toner, as disclosed, for example, in U.S. Pat. No. 4,371,599. In this case, the toner heating during the process of transfer increases the cohesion between the toner particles and its adhesion to the xe2x80x9cbarexe2x80x9d receiving surface of the PCB. However, such a thermal process is characterized by contradictory conditions of the process realization: On one hand, to provide the complete toner transfer, a high heating temperature of the receiving material should be provided (up to 250xc2x0 C.), depending on the melting temperature of the toner and the given technological speed of the process. On the other hand, the photo-semiconductive layers used in photoreceptors are known as very sensitive to high temperatures, such that their parameters are sharply reduced even at temperatures of about 30xc2x0 C.-40xc2x0 C.
Furthermore, under such conditions that the photo-semiconductive surfaces of a photoreceptor directly contact the receiving surface of a rigid metal information carrier, unavoidable mechanical damages of the photo-semiconductive layer and sequential electrical breakage occur. This immediately makes the photo-semiconductive layer useless for further operation. The sources and kinds of damages can be different (pilling, scratches, etc.). For this reason, the thermal transfer based electrophotographic apparatuses usually utilize an intermediate, thermal, rubberized cylinder or a heated belt conveyor (as disclosed in U.S. Pat. No. 5,576,135), which is installed between a photoreceptor and the receiving surface of a final information carrier, and which should be equipped with a thermostatic system. In this case, the operational conditions of the photoreceptor are partially simplified, but the construction of the entire apparatus becomes significantly complicated, and the probability of the image quality degradation increases due to the double transfer process. Furthermore, to provide the required temperature mode for the toner image transfer from a heated drum onto a metal surface possessing high heat-conductivity, this surface should be preliminary heated. However, the combination of the thermal transfer processes with the simultaneous toner image fixation on the metal surface, does not guarantee the sufficient quality of the fixation, owing to the fact that these processes have different optimal temperature modes.
In view of the above, it appears that one of the main reasons limiting the application of an electrophotographic process for the manufacture of PCBs is the absence of an effective system for the toner-image transfer from the photoreceptor surface to the conductive receiving surface. For this reason, none of the existing electrophotography-based techniques for PCB production, is practically applied in industrial PCB production equipment.
The conditions of realization of the transfer process to a conductive surface can be significantly simplified when using a photoreceptor with a protective dielectric coating of the photo-semiconductive layer. This allows for carrying out the direct contact transfer of a toner-image from the surface of the photoreceptor directly onto the conductive surface of an information carrier.
Electrophotographic processes of the latent electrostatic image formation on a photoreceptor with a dielectric coating of the photo-semiconductive layer are known, such as Canon process, Katsuragawa process, Hall process). They are based on a certain sequence of the realization of separate stages of the electrostatic recording. In this case, the exposure process is carried out either by means of the conventional slot image projection technique, or by the beam scanning of the image along the width of the copying region (along the generating line of the cylindrical photoreceptor). Here, the copying process is typically performed in a cyclic mode, the surface of the cylindrical photoreceptor sequentially undergoing all the stages of the electrophotographic process during a single revolution.
However, in some cases, for the parameter optimization, the realization speeds of some stages of the electrophotographic process have to be decreased or increased. Such a problem is solved, for example, when using a multiple-copy production method, in which the process of the latent electrostatic image formation and the process of copy production are timely separated. This is disclosed in U.S. Pat. No. 5,995,795 assigned to the assignee of the present application. The necessary condition for carrying out this method is the maintenance of the potential relief of a latent electrostatic image at the given level during the entire process of multiple-copy production, that is during a certain period of time. This condition is provided when using a photoreceptor with a dielectric coating of the photo-semiconductive layer.
Separation between the processes of electrostatic recording and printing allows for applying non-traditional systems of optical scanning of the copying image for the formation of a latent electrostatic image. However, this can be carried out with correspondingly varying the conditions of the electrostatic recording process and of the entire electrophotographic process.
For example, a system of the image exposure on a photoresist material supported on the outer surface of a rotating cylinder is known, in which the optical image scanning is performed along a helix by moving a recording head along the generating line of the cylinder. Such a system is characterized by the high quality recording of large-format images, and is used in the manufacture of polygraphic printed matrices, as well as phototools for the PCB manufacture.
Various examples of the known systems of laser exposure are disclosed in the following article S. B. Tokes xe2x80x9cApplications of multi-beam acousto-optical modulators in laser-electrophotographic printing and drawing machinesxe2x80x9d, Proceedings, Vol. 396, Advances in Laser Scanning and Recording, Apr. 19-20, 1983, Geneva, Switzerland. These systems utilize optical schemes with longitudinal image scanning along the generating line of the cylindrical photoreceptor, and with transverse image scanning along a helical line on a cylindrical surface covered by a photoresist film.
It is evident that the exposure technique based on a transverse image scanning is not applicable for the traditional electrophotographic processes of the latent electrostatic image formation. This is due to the fact that the optical scanning along a helical line excludes the cyclic nature of the electrophotographic copying process, and the time intervals in sequence and duration of different stages in the process of electrostatic recording at different regions of the photoreceptor surface cannot thereby be maintained.
Thus, to carry out the electrophotographic copying process by applying the optical image scanning along a helix line, two main conditions should be satisfied, as follows:
the timely separation between the process of the latent electrostatic image formation on the photoreceptor surface and the printing process, and
provision of identical conditions for the potential relief formation on all the regions of the photoreceptor surface. Both may be achieved by the use of a cylindrical photoreceptor with a dielectric coating of the photo-semiconductor layer.
There is accordingly a need in the art to facilitate the electrophotographic process of multiple-copy production of a large-format original, by providing a novel method and apparatus for electrophotographic copying utilizing optical image scanning along a helix line.
Thus, according to one broad aspect of the present invention, there is provided a method of electrostatic recording on a cylindrical photoreceptor with a dielectric coating, the method comprising the following steps:
(a) charging the photoreceptor;
(b) exposing the photoreceptor;
(c) discharging the photoreceptor in darkness; and
(d) flood illumination of the photoreceptor;
wherein said exposing is performed by means of an optical image scanning along a helix line.
According to one embodiment of the invention, the charging of the dielectric surface of the photoreceptor is carried out in darkness up to a given potential level during the entire process of image scanning, and after completing the exposure process, the dielectric surface of the photoreceptor is discharged in darkness by an alternating current corona (AC corona). Thereafter, flood illumination of the photosemiconductor layer is carried out.
According to another embodiment of the invention, the photoreceptor first undergoes a preliminary charging in light by the AC corona, and is then exposed in the operational zone of the AC corona. When the exposure is complete, the photosemiconductor layer is illuminated.
According to another aspect of the present invention, there is provided an electrophotographic apparatus for multiple-copy production of original information, the apparatus comprising:
(i) a cylindrical photoreceptor with a dielectric coating of a photosemiconductor layer;
(ii) a charging device for charging the photoreceptor;
(iii) an exposure device for exposing the photoreceptor, said exposure device being of a kind capable of optical scanning of an image of copying information along a helix line;
(iv) a discharging device for discharging the dielectric surface of the photoreceptor;
(v) an illumination device for illuminating the photosemiconductor layer;
(vi) a development device for developing a latent electrostatic image by a toner;
(vii) a transfer device for transferring a toner-image onto a final information carrier;
(viii) a fixation device for image fixation on the copy;
(ix) a cleaning device for cleaning the dielectric surface of the photoreceptor.
Preferably, the charging device is a wire electrizer coupled to a source of alternating voltage with basic bias, defining a given potential level of charging. The discharging device may also be a wire electrizer coupled to a source of alternating voltage.
The apparatus preferably operates in a cyclic mode, and may have the following cycles; optical recording, by simultaneous operation of the charging and exposure devices; revealing a potential relief, by sequential operation of the discharging and illumination devices; multiple-copy production, by operation of the development, transfer and fixation devices; and restoration, by operating neutralizing and cleaning means. In this case, the charging device is accommodated in an exposure zone. The discharging and illumination devices may serve together as the neutralizing means.
Alternatively, the cyclic mode of operation of the apparatus may comprise the following cycles: charging the photoreceptor with simultaneous flood illumination thereof, by simultaneous operation of the charging device and an additional illumination device; optical recording; by simultaneous operation of the discharging and exposure devices; revealing of a potential relief, by operating the illumination device; multiple-copy production, by operation of the development, transfer and fixation devices only; and restoration, by operation of the neutralizing and cleaning means. In this case, the discharging device is accommodated in an exposure zone. The charging device and the additional lighting device may operate during more than one revolution of the photoreceptor.
According to yet another aspect of the present invention, there is provided a method for electrophotographic copying on an electrically conductive information carrier, the method comprising the steps of:
(1) forming a latent electrostatic image on the surface of a photoreceptor;
(2) developing the image by toner;
(3) transferring a toner-image onto the electrically conductive surface of the information carrier; and
(4) fixating the toner-image;
wherein the latent electrostatic image is formed on the photoreceptor laminated by a protecting dielectric coating, and said transferring is carried out by direct electrostatic transfer.
The toner-image transfer from the photoreceptor may be carried out by a contact electrostatic method.
The direct electrostatic transfer is implemented by supplying direct voltage to the electrically conductive information carrier, the supplied voltage having polarity opposite to that of the toner charge and magnitude exceeding a potential level of the electrostatic recording on the dielectric coating of the photoreceptor. Generally, the transferred toner-image may be fixed by a non-contact method.